The determination of the swelling-clay content, especially smectite content, of a formation is important in both the exploration for and the production of hydrocarbons. In exploration, the clay content is useful in the determination of water and hydrocarbon saturations in shaly reservoir formations. During drilling, knowledge of the swelling-clay content is useful in making a determination of the amount of potassium which needs to be added to drilling muds in order to provide wellbore stability. The swelling-clay content also provides information regarding drilling problems such as wellbore instability, stuck pipe, bottom-hole fill, bit balling, mud rings, torque, drag, and solids build-up in the drilling fluid. Completion problems such as formation damage in shaly sands, logging and coring failures, hole washouts, and poor cement jobs sometimes attributable to the excessive clay content of the formation. It is desirable, therefore, to be able to obtain, at the wellsite, timely estimates of the swelling-clay content of earth formations.
Two of the most common methods for determining the swelling-clay content of a sample are the surface area method and the cation exchange capacity (CEC) method. Both of these methods are well established in the art. Briefly, the surface area method correlates the weight increase of a sample exposed to ethylene glycol, ethylene glycol monoethyl ether (EGME), or a similar fluid to the swelling-clay content. See for example, Theng, B. K. G., "The Chemistry of Clay Organic Reactions", John Wiley & Sons, New York, 1974, Chapter 3, for a brief summary of surface area methods used in determining swelling-clay content. The CEC method, on the other hand, correlates the number of exchangeable cations in a sample (cations in the sample that can be replaced by another cation such as barium or ammonium) to the swelling-clay content. See for example, Van Olphen, H., "An Introduction to Clay Colloid Chemistry", Wiley-Interscience, New York, 1977, Chapter 5, for a brief summary of CEC methods used in determining swelling-clay content. However, it is also well established that both of these methods usually require days to complete and cannot easily be done at the wellsite. In addition, these methods can be affected by the presence of other, nonswelling clay minerals, such as zeolites and amorphous silica, and can be sensitive to experimental techniques.
There exists a need, therefore, for a rapid and reliable wellsite method for the determination of the swelling-clay content in shales and shaly formations.
Currently, dielectric measurements are utilized for other, unrelated purposes. For example, dielectric measurements are utilized in logging tools for making determinations of the water and hydrocarbon content in sandstones and carbonates. These logging tools are not designed for making swelling-clay determinations. In addition, these logging tools lose their effectiveness in high-salinity formations.
To the best of Applicants' knowledge, dielectric measurements are not used for making determinations of swelling-clay content. In fact, current art actually dismisses dielectric responses observed between 1-50 MHz in dilute aqueous swelling-clay suspensions as anomalies which vanish with increasing salinity. See for example, Raythatha, R. and Sen, P. N., "Dielectric Properties of Clay Suspensions in the MHz to GHz Range", Journal of Colloid and Interface Science, February 1986, Vol. 109, No. 2, in general, and particularly see pages 305 and 308 wherein it is stated that the electrochemical effects (of swelling-clays) become unimportant at high salinities and the geometrical effects dominate. Therefore, the prior art fails to recognize that dielectric measurements may be utilized for making determinations of the swelling-clay content in shaly formations.